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Intrigued that you think the 19s ride smoother - the 18 would have more sidewall which should make for a nicer ride.
Define "nicer" ride. With more sidewall, depending on the stiffness of the sidewall (which varies within and between mfg.) the car can handle less precisely. If you want a smooth ride, get a caddy
I know my ride is more precise and more predictable...running 19"OZ rims with Yokos.